In a typical Laser In-Situ Keratomeleusis (LASIK) procedure, a microkeratome is used to incise the cornea of a patient and create a flap. The flap is then lifted to expose a bed of stromal tissue which is subsequently ablated using an Excimer laser. The use of a mechanical device, such as a microkeratome, to create a flap has several disadvantages. For example, the creation of a suitable flap with the microkeratome relies heavily on the skill and eye-hand coordination of the surgeon. In such operations, complications can result if the flap is cut improperly or is completely severed from the cornea. In addition, the use of a microkeratome often produces an irregular incision. As a consequence, the bed of tissue that is exposed when the flap is lifted often contains surface irregularities that can create wrinkles when the flap is replaced. These wrinkles can produce undesirable vision deficiencies. As an additional drawback, the incision produced by a microkeratome is typically not substantially dome-shaped or parallel to the anterior surface of the cornea. Because the bed of exposed tissue is not parallel to the anterior surface of the cornea, the laser ablation step can be relatively complicated, is often difficult to control, and can result in an ablation that is somewhat uneven relative to the natural curvature of the cornea. This is especially troublesome when the ablation of a uniform thickness of tissue is prescribed to correct an optical deficiency. For example, the ablation of a uniform volume of tissue is typically prescribed to treat myopia (i.e. nearsightedness), which is a widely occurring condition among the adult population.
As an alternative to using a microkeratome, a laser can be used to create a flap for a LASIK type procedure. For example, a train of laser pulses having pulse durations in the femtosecond range, can be directed to a focal point at a predetermined location within a patient's cornea to photodisrupt tissue at the focal point with precision and accuracy. The photodisruption of tissue by femtosecond lasers results from a process called laser induced optical breakdown (LIOB). Specifically, in the LIOB process, optical breakdown occurs in the laser focus due to the extremely high local electrical field strength that is generated. This field exceeds the binding energy of the valence electrons to their atoms, resulting in the generation of a microplasma, gas bubbles and shockwaves.
An example of a procedure that uses a pulsed laser beam focused at a predetermined, subsurface location within a patient's cornea is disclosed in U.S. Pat. No. 4,907,586, which issued to Bille et al. for an invention entitled “Method for Reshaping the Eye”. In greater detail, the above-cited Bille patent discloses the use of a pulsed laser beam for subsurface photodisruption of intrastromal tissue. Unlike the Excimer laser used after flap creation in the conventional LASIK procedure, the pulsed laser beam, as disclosed by Bille, penetrates corneal tissue and can be focused at a point below the surface of the cornea to photodisrupt stromal tissue at the focal point.
The ability to reach a subsurface location without necessarily providing a physical pathway allows for volumes of stromal tissue to be photodisrupted having complicated shapes while minimizing the amount of total tissue disrupted. To create these shapes, the laser beam is first directed to a focal point (using suitable optics to include a cutting lens) at a target location corresponding to a point on the desired volume to be photodisrupted. After at least one pulse is delivered to the focal point to ablate the tissue there, the focal point is moved (i.e. scanned) to another point in the prescribed volume. At the new location, at least one pulse is delivered and the process of scanning and ablating is continued until the entire prescribed volume of tissue is photodisrupted.
From the above discussion, it is apparent that the accuracy, agility and speed of the scanning sub-system can have a large impact on the overall performance of the surgical system. Stated another way, the scanning system is responsible for accurately and very quickly moving the focal point from one prescribed location to another. This demand on the scanning system can be especially acute when scanning is required in all three dimensions (i.e. a movement from one location to the next requires a simultaneous movement in an “x”, “y” and “z” direction). In an effort to simplify this requirement, one technique has been developed which requires the scanning system to scan in only two (rather than three) mutually orthogonal directions. For this technique, the cornea is first applanated to obviate the need for scanning movements in the z direction. This is done by conforming the anterior surface of the cornea against a substantially flat applanating lens. Next, a planar volume of subsurface tissue parallel to the applanating lens is ablated. With the use of a specialized cutting lens having no field curvature, the planar volume can be ablated with only two-dimensional scanning (i.e. scan movements normal to the applanating lens surface are not required). However, as developed further below, this technique can result in several rather severe and undesirable consequences.
Although scanning is simplified, flattening of the cornea can have undesirable consequences. For example, flattening of the cornea can result in severe discomfort for the patient. Moreover, flattening of the cornea can increase the intraocular pressure to dangerously high levels. Lastly, but perhaps of equal importance, severe flattening of the cornea can distort the three-dimensional architecture of the corneal lamellae. The result of this distortion is that an incision that is made while the cornea is severely flattened, changes shape in an unpredictable way when the cornea is relaxed.
For the case where scanning the three mutually orthogonal directions is contemplated, a number of factors can impact system performance. Among these factors involves the scan distance that the focal point must be moved from one ablation point to the next. As this scan distance increases, larger scan movements are required. As recognized by the present invention, scan accuracy is typically proportional to scan distance. Accordingly, by reducing the scan distance, scan accuracy can be improved. In addition, relatively large scan movements often result in increased wear on the scanner system components. This wear can decrease the life of the scanner system. Lastly, short scan movements can be executed faster than large scan movements. Thus, it is desirable to use an ablation scan pattern which minimizes scan movements between consecutively ablated spots to thereby quicken the procedure. In somewhat simpler terms, for a given scan response time, shorter scan movements can accommodate the use of a faster laser pulse repetition rate and result in a shorter overall procedure time.
In light of the above, it is an object of the present invention to provide systems and methods suitable for the purposes of photodisrupting subsurface ablations having relatively complex shapes such as a substantially dome-shaped layer of stromal tissue. It is another object of the present invention to provide methods for creating a flap that can be used as part of a LASIK type procedure, and particularly, a flap which exposes a surface for ablation that is substantially parallel to the natural, anterior surface of cornea. It is yet another object of the present invention to provide a method for photodisrupting a preselected volume of tissue using a series of relatively small scan movements. Yet another object of the present invention is to provide a method for photodisrupting stromal tissue at a plurality of subsurface focal points which is easy to use, relatively simple to implement, and comparatively cost effective.